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Olivia Martius

Abstract

This study presents a 5-yr climatology of 7-day back trajectories started from the Northern Hemisphere subtropical jet. These trajectories provide insight into the seasonally and regionally varying angular momentum and potential vorticity characteristics of the air parcels that end up in the subtropical jet. The trajectories reveal preferred pathways of the air parcels that reach the subtropical jet from the tropics and the extratropics and allow estimation of the tropical and extratropical forcing of the subtropical jet.

The back trajectories were calculated 7 days back in time and started every 6 h from December 2005 to November 2010 using the Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) dataset as a basis. The trajectories were started from the 345-K isentrope in areas where the wind speed exceeded a seasonally varying threshold and where the wind shear was confined to upper levels.

During winter, the South American continent, the Indian Ocean, and the Maritime Continent are preferred areas of ascent into the upper troposphere. From these areas, air parcels follow an anticyclonic pathway into the subtropical jet. During summer, the majority of air parcels ascend over the Himalayas and Southeast Asia.

Angular momentum is overall well conserved for trajectories that reach the subtropical jet from the deep tropics. In winter and spring, the hemispheric-mean angular momentum loss amounts to approximately 6%; in summer, it amounts to approximately 18%; and in fall, it amounts to approximately 13%. This seasonal variability is confirmed using an independent potential vorticity–based method to estimate tropical and extratropical forcing of the subtropical jet.

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Olivia Martius and Heini Wernli

Abstract

Tropical, subtropical, and extratropical dynamical processes govern the synoptic-scale evolution of the subtropical jet stream(s) over Africa. However, the relative importance of the respective effects is still under debate and is the focus of this study. Interim ECMWF Re-Analysis (ERA-Interim) data are used to calculate backward trajectories from the subtropical jet over Africa during winter 2005/06. The trajectories allow for studying the jet dynamics from both a potential vorticity (PV) and an angular momentum point of view and for linking the two theoretical frameworks.

Three cases of synoptic-scale Rossby wave breaking in the extratropics and subtropics are presented in detail. They illustrate basic flow configurations where (i) the subtropical jet is mainly forced by tropical dynamics, (ii) extratropical forcing contributes substantially to the jet acceleration, and (iii) strong diabatic processes in the subtropics impact the jet.

The case study results are then generalized for the entire winter season. The main findings are as follows: (i) Approximately 41% of the trajectories reach the subtropical jet from the deep tropics and for these trajectories the nonconservation of angular momentum M due to eddy forcing leads to a decrease of M by about 5%. (ii) A nonnegligible fraction of roughly 18% of the trajectories reaches the subtropical jet from the extratropics. (iii) Wave breaking is instrumental for bringing extratropical, high-PV air southward. (iv) Diabatic processes in the subtropics have a negligible direct effect on the upper-level PV. This is in contrast to observations from the extratropics and might be the consequence of the small planetary vorticity in the tropics and subtropics.

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Paraskevi Giannakaki and Olivia Martius

Abstract

An accurate representation of synoptic-scale Rossby waves in numerical weather forecast models is very important as these waves are closely linked to weather formation at the surface. Enhanced potential vorticity (PV) gradients at the tropopause levels act as waveguides for synoptic-scale Rossby waves, so spatial errors in the waveguides imply errors in the amplification and propagation of Rossby waves. This paper focuses on evaluating the forecast representation of these waveguides and presents an object-based forecast verification tool. In both forecast and the verification data, Rossby waveguide objects are defined based on enhanced PV gradient fields on isentropic surfaces. The tool automatically pairs the complex objects, compares their properties, and assesses the number of objects without a matching partner in either the forecast or the reanalysis. In the last step, error measures are calculated for the area and the location of the objects. As proof-of-concept application of the method for the year 2008, five lead times of the Integrated Forecast System (IFS) from the ECMWF are compared with the ECMWF reanalysis dataset. The majority of the waveguide objects are found to be in the correct position, and there are no systematic positional errors; however, the forecast objects and hence the areas of enhanced PV gradients are smaller.

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Alexandre Tuel and Olivia Martius

Abstract

Temporal clustering of extreme precipitation (TCEP) at subseasonal time scales often results in major impacts on humans and ecosystems. Assessment and mitigation of the risk of such events requires characterization of their weather/climate drivers and their spatial dependence. Here, we introduce a regionalization method that identifies coherent regions in which the likelihood of subseasonal TCEP exhibits similar dependence to large-scale dynamics. We apply this method to each season in the Northern Hemisphere using ERA5 reanalysis data. The analysis yields spatially coherent regions, primarily at high latitudes and along the eastern margins of ocean basins. We analyze the large-scale and synoptic conditions associated with TCEP in several of the identified regions, in light of three key ingredients: lifting, moisture availability, and persistence in synoptic conditions. We find that TCEP is often directly related to distinct cyclone and blocking frequency anomalies and upper-level wave patterns. Blocking and associated Rossby wave breaking are particularly relevant at high latitudes and midlatitudes. At upper levels, meridional wave patterns dominate; however, in western Europe and parts of North America, TCEP is sometimes associated with zonally extended wave patterns. The flow features associated with TCEP in the eastern Pacific and eastern Atlantic Oceans exhibit similarities. For some regions, moisture flux anomalies are present during clustering episodes whereas in others forced lifting alone is sufficient to trigger heavy precipitation. Our results provide new information on the dynamics and spatial dependence of TCEP that may be relevant for the subseasonal prediction of clustering episodes.

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Olivia Martius, Kathrin Wehrli, and Marco Rohrer

Abstract

Three sets of model experiments are performed with the Community Earth System Model to study the role of soil moisture anomalies as a boundary forcing for the formation of upper-level Rossby wave patterns during the Southern Hemisphere summer. In the experiments, soil moisture over Australia is set to ±1 standard deviation (STD) of an ERA-Interim-derived soil moisture reconstruction for the years 2009–16 and 50 ensemble members are run. The local response is a positive heating anomaly in the dry simulations that results in a thermal low–like circulation anomaly with an anomalous surface low and upper-level anticyclone. Significant differences in convective rainfall over Australia are related to differences in convective instability and associated with changes in near-surface moisture and moisture advection patterns. A circum-hemispheric flow response is identified both in the upper-level flow and in the surface storm tracks that overall resembles a positive southern annular mode–like flow anomaly in the dry simulations. The structure of this atmospheric response strongly depends on the background flow. The results point to a modulation of the hemispheric flow response to the forcing over Australia by El Niño–Southern Oscillation. Significant changes of precipitation over the Maritime Continent and South Africa are found and significant differences in the frequency of surface cyclones are present all along the storm tracks.

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Olivia Martius, Kathrin Wehrli, and Marco Rohrer

Abstract

Three sets of model experiments are performed with the Community Earth System Model to study the role of soil moisture anomalies as a boundary forcing for the formation of upper-level Rossby wave patterns during Southern Hemisphere summer. In the experiments, soil moisture over Australia is set to ±1STD of an ERA-Interim reanalysis derived soil moisture reconstruction for the years 2009 to 2016 and 50 ensemble members are run. The local response is a positive heating anomaly in the dry simulations that results in a thermal low-like circulation anomaly with an anomalous surface low and upper-level anticyclone. Significant differences in convective rainfall over Australia are related to differences in convective instability and associated with changes in near surface moisture and moisture advection patterns. A circum-hemispheric flow response is identified both in the upper-level flow and in the surface storm tracks that overall resembles a positive Southern Annular Mode-like flow anomaly in the dry simulations. The structure of this atmospheric response strongly depends on the background flow. The results point to a modulation of the hemispheric flow response to the forcing over Australia by the El Niño Southern Oscillation. Significant changes of precipitation over the Maritime continent and South Africa are found and significant differences in the frequency of surface cyclones are present all along the storm tracks.

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Matthias Röthlisberger, Olivia Martius, and Heini Wernli

Abstract

A climatology of Rossby wave initiation (RWI) events on the Northern Hemisphere midlatitude jet is compiled by applying an objective RWI identification algorithm to the ERA-Interim dataset. In winter, RWI events occur most frequently over the northwestern Pacific and less often over the North Atlantic. In summer, the total number of RWI events is lower than in winter and the North Pacific RWI region shifts toward the Tibetan Plateau. Composite analysis of the large-scale flow prior to, during, and after winter North Pacific RWI events shows an upstream wave train propagating across Asia on the Arctic waveguide prior to RWI. The composite wave forms on a relatively weak zonal jet streak, exhibits a baroclinic structure, and is strongly amplified by latent heat release in the warm conveyor belt of a deepening surface cyclone. Moreover, the wave forms in a region of large-scale upper-level deformation, upstream of a preexisting ridge. Further, active tropical convection affects the longitude where RWI occurs and thus acts as a geographical anchor for RWI. Individual RWI events are characterized by preferred combinations of these composite features: a strong surface cyclone tends to occur in concert with strong latent heating and a pronounced positive PV anomaly aloft. A second group of co-occurring features contains active tropical convection, a strengthened subtropical anticyclone, and the downstream ridge. These feature groups might be regarded as idealized archetypal RWI scenarios, although numerous intermediate events exist where features from both groups occur together.

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Christian Franzke, Tim Woollings, and Olivia Martius

Abstract

The persistent regime behavior of the eddy-driven jet stream over the North Atlantic is investigated. The North Atlantic jet stream variability is characterized by the latitude of the maximum lower tropospheric wind speed of the 40-yr ECMWF Re-Analysis (ERA-40) data for the period 1 December 1957–28 February 2002. A hidden Markov model (HMM) analysis reveals that the jet stream exhibits three persistent regimes that correspond to northern, southern, and central jet states. The regime states are closely related to the North Atlantic Oscillation and the eastern Atlantic teleconnection pattern. The regime states are associated with distinct changes in the storm tracks and the frequency of occurrence of cyclonic and anticyclonic Rossby wave breaking. Three preferred regime transitions are identified, namely, southern to central jet, northern to southern jet, and central to northern jet. The preferred transitions can be interpreted as a preference for poleward propagation of the jet, but with the southern jet state entered via a dramatic shift from the northern state. Evidence is found that wave breaking is involved in two of the three preferred transitions (northern to southern jet and central to northern jet transitions). The predictability characteristics and the interannual variability in the frequency of occurrence of regimes are also discussed.

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Richard W. Moore, Olivia Martius, and Thomas Spengler

Abstract

The 40-yr ECMWF Re-Analysis (ERA-40) data are combined with a number of novel climatologies to conduct a comprehensive examination of the response of the subtropical and extratropical atmosphere over the Pacific basin to an evolving Madden–Julian oscillation (MJO) event. The adopted approach constitutes a symbiosis of a climatological analysis during the Northern Hemisphere winter from 1979 to 2002 and a case study analysis of a distinct MJO event that occurred in January–February 1993. The former is designed to obtain the general characteristics observed during a composite MJO life cycle, while the latter is used to provide insight into the instantaneous mechanisms responsible for the observed composite evolution.

A primary component of the study involves the diagnosis of anomalous wave breaking activity in response to MJO forcing in the form of tropical convection and/or upper-level divergence. Wave breaking events are separated by their characteristic life cycles: LC1 (anticyclonic) and LC2 (cyclonic) events. Statistically significant anomalies in wave breaking activity are found to be prevalent during the composite MJO event. Furthermore, the dynamical distinction between LC1 and LC2 wave breaking is useful in that the two different characteristic life cycles exhibit significantly different anomalous behavior during the MJO.

Statistically significant variability is also identified in both the subtropical and extratropical flow and atmospheric blocking and surface cyclone frequency. These data, taken in conjunction with the observed evolution of the 1993 MJO event, provide a relatively coherent picture of the response of the atmosphere to MJO forcing. A schematic representation of the evolution is presented.

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Erica Madonna, Heini Wernli, Hanna Joos, and Olivia Martius

Abstract

A global climatology of warm conveyor belts (WCBs) is presented for the years 1979–2010, based on trajectories calculated with Interim ECMWF Re-Analysis (ERA-Interim) data. WCB trajectories are identified as strongly ascending air parcels (600 hPa in 2 days) near extratropical cyclones. Corroborating earlier studies, WCBs are more frequent during winter than summer and they ascend preferentially in the western ocean basins between 25° and 50° latitude. Before ascending, WCB trajectories typically approach from the subtropics in summer and from more midlatitude regions in winter. Considering humidity, cloud water, and potential temperature along WCBs confirms that they experience strong condensation and integrated latent heating during the ascent (typically >20 K). Liquid and ice water contents along WCBs peak at about 700 and 550 hPa, respectively. The mean potential vorticity (PV) evolution shows typical tropospheric values near 900 hPa, followed by an increase to almost 1 potential vorticity unit (PVU) at 700 hPa, and a decrease to less than 0.5 PVU at 300 hPa. These low PV values in the upper troposphere constitute significant negative anomalies with amplitudes of 1–3 PVU, which can strongly influence the downstream flow. Considering the low-level diabatic PV production, (i) WCBs starting at low latitudes (<40°) are unlikely to attain high PV (due to weak planetary vorticity) although they exhibit the strongest latent heating, and (ii) for those ascending at higher latitudes, a strong vertical heating gradient and high absolute vorticity are both important. This study therefore provides climatological insight into the cloud diabatic formation of significant positive and negative PV anomalies in the extratropical lower and upper troposphere, respectively.

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